The invention relates to a castable steel composition including a martensitic matrix structure and methods for forming such composition.
Iron based materials are commonly heat treated to improve strength. Typically, carbon steels are rapidly quenched from the face centered austenitic phase to form martensite. This resulting structure is characterized as a body-centered tetragonal lattice (distorted body-centered cubic) with the degree of distortion being proportional to the amount of trapped carbon.
Martensite is exceptionally strong, hard and brittle. Because it lacks good toughness and ductility, a heat treatment of between about 300xc2x0-1200xc2x0 F. known as tempering is usually employed to improve toughness by redistributing some of the carbon from the solution to yield a mixture of stable ferrite and cementite phases.
Both the quenching and tempering stages are time and energy intensive. It is therefore an object of the invention to provide a strong, hard, ductile martensitic steel in which the quenching and tempering steps are eliminated.
The present invention is directed to a specialty alloy specifically formulated to produce an as-cast structure comprised of a ductile fine grain tempered martensite phase exhibiting a hardness of HRC 40 to HRC 50. The ductile martensite phase is a matrix consisting primarily of iron, chromium, nickel and molybdenum. A fine grained microstructure of fully tempered martensite forms, upon cooling from the liquid phase, as a casting is slow cooled from a pour. The alloy is unique, in that it forms martensite at a high temperature during the cooling cycle, and then the residual heat in the casting tempers the martensite such that the resulting room temperature structure is one of essentially all fine tempered martensite.
As set forth above, typically alloys must be quenched rapidly in air, oil or water to a relatively low temperature above or below room temperature in order to form the brittle fresh martensite structure. The fresh martensite structure then needs to be reheated to a tempering temperature, typically from around 300xc2x0-1200xc2x0 F. in order to form the tougher more ductile structure of tempered martensite.
The alloy of the present invention forms fresh martensite at a sufficiently high enough temperature during the cooling cycle of the casting so that this fresh martensite becomes tempered during the remaining cooling cycle of the casting. This results in a single pour cycle that produces a uniform tempered martensite throughout the casting without subsequent heat treatment. This alloy must cool slowly to adequately form the tempered martensite structure. That is why the alloy is so well suited to the casting process, which is a naturally slow cooling process.
The present invention provides many benefits. The alloy microstructure that forms during cooling results in an alloy that has excellent strength, ductility, toughness and wear resistance, due to the formation of the fine grained martensitic structure. This combination of properties and alloy structure were formerly only available by performing a quench and tempering heat treatment process. Also, the combination of elements, primarily chromium, nickel. And molybdenum, gives this alloy excellent resistance to corrosion.